The mechanism of action of ristocetin-induced aggregation of platelets in the presence of the von Willebrand factor will be studied by analyzing the effects of chemical and enzymatic modifications of the platelet surface. The electrostatic interactions between platelets, von Willebrand factor and ristocetin will be assessed by measuring electrophoretic mobility of platelets under different conditions. Platelet binding studies using labeled ristocetin or its equivalent will give additional information on these interactions.